Android: back button for Learn subpages

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Summary

Measured two reported main-window parity gaps against app-gnome.

Confirmed — Android Learn back button. app-gnome shows a visible
go-previous-symbolic back button in the header when a Learn subpage
(Tutorial/Examples) is open (main.window.blp:107, wired in
main.window.ts via hasVisibleSubpage/updateLearnBackButtonVisibility).
app-web has the same (main-window.ts:114,283-286,516-518). Android's
Adw.NavigationView port has no built-in per-page header/back button
(unlike native libadwaita), and the app only handled the hardware/system
back key (onBack in views/main/learn.ts) — there was no on-screen
affordance at all.

Fix: Learn (views/main/learn.ts) now exposes hasVisibleSubpage and
a subpage-changed event (via @learn6502/core's EventDispatcher),
mirroring the GNOME GObject property and the web CustomEvent. The
shell (views/main.ts) packs a go-previous Gtk.Button into the
header's start slot, wired to learnView.navigateBack(), visible only
while the Learn tab is active and a subpage is open (Android has no
desktop/three-column mode, so that's the whole condition — the GNOME
twin's mobile branch).

Not confirmed as a parity fix — Web "unsaved changes" dot. The
dot widget itself really is absent from app-web's header. But in
app-gnome the dot is the tail of a full Open/Save-file feature
(Gtk.FileDialog + Gio.File, services/file.service.ts) that
tracks a currentFile and clears the flag on save. app-web has
no file-open/save feature anywhere — no menu entry, no
FileService implementation, not even a "Save" string in the whole
package. Building a faithful counterpart means designing app-web's
entire file-persistence story (there is no File System Access API
today; a <input type=file> + Blob-download approach would need to
be chosen), which is a product decision beyond a header-widget parity
fix. Left untouched; flagging for its own task.

Test plan

• gjsify format / gjsify format --check — clean, no drift
• ns build android (after gjsify workspace @learn6502/core build
  to produce packages/core/dist) — Vite bundle + Gradle build
  succeeded, app-debug.apk produced
• Not run: on-device/emulator verification that the button
  actually appears/disappears and pops the nav stack — only
  statically verified via the wiring above

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