Keep a place, and come back to it

Restores the commit reverted off the routing branch, which is where it was
committed by mistake. The change is unmodified; only its branch is.

The map had no memory. Every visit started at the same overview, and a system
worth returning to had to be found again by name each time. A mark on the
readout and on a body's panel now keeps it, a Bookmarks tab lists what has been
kept, and choosing one goes there — a star by flying into its system, a body by
opening its page.

Local storage, not an account. This map asks nobody to sign in, and a list of
stars somebody liked is not worth a server. Every read of that store is
defensive, because it is a string a person can edit, another tab can write, and
a browser can refuse to hand over at all: a bad entry is skipped rather than
losing the rest, duplicates are collapsed since two entries for one place would
each toggle the other's control, the list is bounded so a hand-edited store
cannot decide how much this renders, and where storage is denied outright the
bookmarks still work for the visit — they just do not outlive it.

The name is stored alongside the id rather than looked up, so the list reads
before the catalogues have loaded, and a bookmark to something a later
catalogue no longer holds still says what it was instead of decaying into a
bare number.

The tab is offered even when it is empty, and says what the mark does: a tab
that appears only once you have already found the feature is a tab that never
taught anyone anything.

Choosing a kept place hands the panel back to the readout, the same move as
choosing a search result and for the same reason. That behaviour is what the
end-to-end spec caught missing — the readout it asserted on did not exist,
because the panel just used was still covering it.

Verified: build clean, 587/587 unit, 11/11 end-to-end, design detector clean,
screenshots at 1440x900 and 390x844.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jxMkwA2rbicdGxHosecYi

src/app/shared/state/bookmarks.store.ts

@@ -0,0 +1,111 @@
+import { Injectable, signal } from '@angular/core';
+
+/**
+ * A place someone chose to keep. Either a star, which is a system to fly into, or a body, which
+ * is a page to open — the two things the map lets you arrive at.
+ */
+export interface Bookmark {
+  readonly kind: 'star' | 'body';
+  /** A HYG star id, or a `bodies.json`/`exoplanets.json` id. */
+  readonly id: number | string;
+  /**
+   * The name as it read when it was kept. Stored rather than looked up, so the list can be
+   * shown before the catalogues have loaded — and so a bookmark to something a later catalogue
+   * no longer holds still says what it was rather than becoming a bare id.
+   */
+  readonly name: string;
+}
+
+const STORAGE_KEY = 'star-map.bookmarks';
+
+/**
+ * How many are kept. Not a limit anyone will reach by hand — it is a bound on what a corrupted
+ * or hand-edited store can make the app render, and on what is written back.
+ */
+const MAX_BOOKMARKS = 200;
+
+/** `${kind}:${id}`, since a star id and a body id are different kinds of thing. */
+function keyOf(kind: Bookmark['kind'], id: number | string): string {
+  return `${kind}:${id}`;
+}
+
+function isBookmark(value: unknown): value is Bookmark {
+  if (typeof value !== 'object' || value === null) {
+    return false;
+  }
+  const candidate = value as Partial<Bookmark>;
+  return (
+    (candidate.kind === 'star' || candidate.kind === 'body') &&
+    (typeof candidate.id === 'number' || typeof candidate.id === 'string') &&
+    typeof candidate.name === 'string'
+  );
+}
+
+/**
+ * The places kept between visits, in this browser and nowhere else.
+ *
+ * Local storage rather than an account: the map asks nobody to sign in, and a list of stars
+ * somebody liked is not worth a server. Every read of it is defensive — the store is a string
+ * a user can edit, another tab can write, and a browser can refuse to give at all — and a
+ * failure to read or write one is never allowed to take the map down with it.
+ */
+@Injectable({ providedIn: 'root' })
+export class BookmarksStore {
+  private readonly kept = signal<readonly Bookmark[]>(this.read());
+
+  /** Most recently kept first, which is the order they are useful in. */
+  readonly bookmarks = this.kept.asReadonly();
+
+  has(kind: Bookmark['kind'], id: number | string): boolean {
+    return this.kept().some((bookmark) => bookmark.kind === kind && bookmark.id === id);
+  }
+
+  /** Keeps a place, or drops it if it was already kept. Returns whether it is kept now. */
+  toggle(bookmark: Bookmark): boolean {
+    const kept = this.has(bookmark.kind, bookmark.id);
+    this.write(kept ? this.kept().filter((other) => !(other.kind === bookmark.kind && other.id === bookmark.id)) : [bookmark, ...this.kept()].slice(0, MAX_BOOKMARKS));
+    return !kept;
+  }
+
+  remove(kind: Bookmark['kind'], id: number | string): void {
+    this.write(this.kept().filter((bookmark) => !(bookmark.kind === kind && bookmark.id === id)));
+  }
+
+  private write(bookmarks: readonly Bookmark[]): void {
+    this.kept.set(bookmarks);
+    try {
+      localStorage.setItem(STORAGE_KEY, JSON.stringify(bookmarks));
+    } catch {
+      // Full, disabled, or private-mode storage. The list still works for this visit; it just
+      // will not outlive it, which is a smaller loss than the alternative of failing here.
+    }
+  }
+
+  private read(): Bookmark[] {
+    let raw: string | null = null;
+    try {
+      raw = localStorage.getItem(STORAGE_KEY);
+    } catch {
+      return [];
+    }
+    if (!raw) {
+      return [];
+    }
+    try {
+      const parsed: unknown = JSON.parse(raw);
+      if (!Array.isArray(parsed)) {
+        return [];
+      }
+      // Filtered rather than rejected wholesale: one bad entry should not lose the others, and
+      // deduplicated because two entries for one place would each toggle the other's control.
+      const seen = new Set<string>();
+      return parsed
+        .filter(isBookmark)
+        .filter((bookmark) => !seen.has(keyOf(bookmark.kind, bookmark.id)) && seen.add(keyOf(bookmark.kind, bookmark.id)))
+        .slice(0, MAX_BOOKMARKS)
+        .map(({ kind, id, name }) => ({ kind, id, name }));
+    } catch {
+      return [];
+    }
+  }
+}