feat(graphics): smart GPU preference with boot watchdog and crash recovery

Solves the long-running performance complaints (especially Optimus
laptops where the dGPU silently hijacks WebKit). The end-to-end shape:

- graphics_detection enumerates physical GPUs from /sys/class/drm and
  cross-references EGL vendor JSONs under /usr/share/glvnd/egl_vendor.d
  to expose only stacks the running process can actually use. Works
  inside Flatpak/Snap because /sys is still mounted read-only and the
  sandbox-visible EGL vendor JSONs are the right thing to read.
- preferred_gpu column in graphics_settings.db (string: auto, integrated,
  discrete, software, or a specific PCI slot id). Migrated with
  ALTER TABLE ... DEFAULT 'auto'.
- env_vars_for_preferred_gpu returns Set/Unset actions applied at startup
  before WebKit init. Integrated/AMD path explicitly unsets
  __NV_PRIME_RENDER_OFFLOAD so KDE/Optimus sessions that pre-set it to 1
  no longer override the EGL pin and yank rendering back onto NVIDIA.
- Auto resolver: on hybrid systems (iGPU + dGPU both usable), Auto pins
  to the iGPU. Single-GPU and no-GPU systems remain no-op. The dropdown
  surfaces what Auto resolved to ("Auto (Intel Graphics)") so the user
  sees the decision without picking explicitly.
- boot_watchdog writes a pending.json before WebKit init listing the
  risky knobs in effect. Frontend calls v2_mark_boot_succeeded after
  first paint to clear it and promote to last-good.json. If the next
  launch sees pending without a success ack, it blames matching fields,
  reverts them in-DB, and increments per-knob failure counters; two
  consecutive failures lock the knob until the user clears it.
- Settings > Graphics gets a Rendering GPU dropdown (between HW accel
  and DMA-BUF) and a crash-recovery banner that lets the user re-enable
  any auto-disabled knob. Restart-required affordance shared with the
  other risky toggles.
- i18n keys for the dropdown, banner copy, and autoWithTarget label
  across all five locales.

cc

src/routes/+page.svelte

@@ -4734,6 +4734,16 @@
     // Bootstrap app (theme, mouse nav, Last.fm restore)
     const { cleanup: cleanupBootstrap } = bootstrapApp();
 
+    // Tell the boot watchdog that the UI mounted successfully. This
+    // clears the pending-graphics-init marker the backend wrote at
+    // startup; if we never reach this line (WebKit crashed during
+    // graphics init) the next launch sees the stale marker and
+    // auto-reverts the offending risky setting. Idempotent and safe to
+    // call on every mount — backend just removes the file if present.
+    void invoke('v2_mark_boot_succeeded').catch((err) => {
+      console.warn('[BootWatchdog] mark_boot_succeeded failed:', err);
+    });
+
     void initDiscordRpc();
 
     // Window-title preference: load from localStorage and subscribe so that