feat: upgrade to gnome 51

Adds GNOME Shell 51 to shell-version and moves the build onto the
GNOME 51 type packages.

- switch to @girs/gjs 5.5.0 and the -51 namespace packages (shell/st/meta
  plus gio/glib/gobject/gtk/adw)
- drop @girs/gnome-shell 49, the pinned @girs/gjs 4.0.0-beta.38 and the 16
  pnpm overrides it needed. The umbrella is still GNOME 50 and pulls St-18
  and Meta-18 types that clash with the -51 packages
- types/: hand-written declarations for the two Shell JS modules this
  extension imports (extension.js and prefs.js). Upstream has these for 51
  in 51.0.0-alpha.1, but that is not published to npm yet, so declaring the
  handful of symbols we use keeps installs registry-only
- TypeScript 6.0.3 rather than 7: typescript-eslint requires <6.1.0 and
  ts-jest requires <7. TS 6 also needs an explicit rootDir (TS5011)
- eslint 10.12, @eslint/js 10.0.1, eslint-plugin-jsdoc 65.3.4 and
  typescript-eslint 8.71.2

types/shell-globals.d.ts

@@ -0,0 +1,20 @@
+/**
+ * GNOME Shell runtime globals.
+ *
+ * The `global` object is created by the shell before an extension is loaded and
+ * has no `.gir`, so it is not covered by the generated `@girs/*-51` packages.
+ * Upstream ships these in `@girs/gnome-shell`, but its GNOME 51 build
+ * (51.0.0-alpha.1) is unpublished, and v50 pulls St-18/Meta-18 types that
+ * collide with the -51 packages. Declaring the few symbols we use keeps the
+ * project on GNOME 51 types and installable from the registry.
+ *
+ * @see https://gitlab.gnome.org/GNOME/gnome-shell/-/blob/main/js/ui/environment.js
+ */
+import type Shell from "@girs/shell-51";
+
+declare global {
+    /** Global shell object created by GNOME Shell on startup. */
+    const global: Shell.Global;
+}
+
+export {};

types/shell-modules.d.ts

@@ -0,0 +1,86 @@
+/**
+ * Ambient declarations for the GNOME Shell JavaScript modules this extension
+ * imports.
+ *
+ * Shell's JavaScript API has no `.gir`, so generated `@girs/*` types do not
+ * cover it. Upstream maintains those types in `@girs/gnome-shell`, but its
+ * GNOME 51 support (51.0.0-alpha.1) is unpublished on npm, and v50 depends on
+ * St-18/Meta-18 types that conflict with the -51 packages this project uses.
+ * Only the symbols actually imported here are declared; extend as needed.
+ *
+ * Signatures follow GNOME Shell 51:
+ * @see https://gitlab.gnome.org/GNOME/gnome-shell/-/blob/main/js/extensions/extension.js
+ * @see https://gitlab.gnome.org/GNOME/gnome-shell/-/blob/main/js/extensions/prefs.js
+ */
+
+declare module "resource:///org/gnome/shell/extensions/extension.js" {
+    import type Gio from "@girs/gio-2.0";
+
+    /** Parsed contents of the extension's `metadata.json`. */
+    export interface ExtensionMetadata {
+        uuid: string;
+        name: string;
+        description: string;
+        "shell-version"?: string[];
+        "settings-schema"?: string;
+        "gettext-domain"?: string;
+        url?: string;
+        [key: string]: unknown;
+    }
+
+    /**
+     * Base class for an extension's `extension.js` default export.
+     *
+     * GNOME Shell 51 requires `disable()` to stay synchronous; returning a
+     * promise now throws.
+     */
+    export class Extension {
+        constructor(metadata: ExtensionMetadata);
+
+        readonly metadata: ExtensionMetadata;
+        readonly uuid: string;
+        readonly path: string;
+
+        /** Returns a {@link Gio.Settings} for the extension's schema. */
+        getSettings(schema?: string | null, subdir?: string | null): Gio.Settings;
+
+        /** Marks the extension's settings as translatable. */
+        translateConfig(): void;
+
+        /** Opens the extension's preferences dialog, if it has one. */
+        openPreferences(): void;
+
+        enable(): void;
+        disable(): void;
+    }
+}
+
+declare module "resource:///org/gnome/Shell/Extensions/js/extensions/prefs.js" {
+    import type Gio from "@girs/gio-2.0";
+    import type Adw from "@girs/adw-1";
+
+    /** Marks a string for translation and returns the translation. */
+    export function gettext(str: string): string;
+
+    /** Pluralised form of {@link gettext}. */
+    export function ngettext(fid: string, fidPlural: string, n: number): string;
+
+    /** Context-sensitive form of {@link gettext}. */
+    export function pgettext(context: string, str: string): string;
+
+    /** Base class for an extension's `prefs.js` default export. */
+    export class ExtensionPreferences {
+        readonly metadata: Record<string, unknown>;
+        readonly uuid: string;
+        readonly path: string;
+
+        /** Returns a {@link Gio.Settings} for the extension's schema. */
+        getSettings(schema?: string | null, subdir?: string | null): Gio.Settings;
+
+        /** Marks the extension's settings as translatable. */
+        translateConfig(): void;
+
+        /** Called by the shell to populate the preferences window. */
+        fillPreferencesWindow(window: Adw.PreferencesWindow): Promise<void> | void;
+    }
+}