firefox-fourier: flatten patches to top-level (makepkg has no subdir support)

Follow-up to #10. The subdir layout I assumed worked turned out to not
work — per 'man PKGBUILD':

  The filename in the array must NOT include any path components like ./

makepkg's source-staging only honors basenames; the patches/ subdir
references staged the files but the basename-lookup at apply time
failed with the same 'not found in build directory' error that #9 hit.

I copied the chromium-fourier convention thinking it was proven, but
chromium-fourier has no CI either and likely has the same latent
breakage (separate issue, not in this PR's scope).

The actually-working pattern is the mpv-fourier one: patches at the
top level next to the PKGBUILD. mpv-fourier iter2 just built green
on CI (#92) with that layout.

This commit:
- Moves all 8 patches (5 fourier + 3 arch upstream) from patches/
  to arch/firefox-fourier/ via git mv (preserves blame).
- Removes the now-empty patches/ subdir.
- Strips 'patches/' prefix from source array entries.
- Strips '/patches' from prepare() patch -i paths.

No semantic change to the patch content or apply order.

pkgrel 3 -> 4.

arch/firefox-fourier/0004-prefs-v4l2-request.patch

@@ -0,0 +1,34 @@
+From: Markus Fritsche <mfritsche@reauktion.de>
+Subject: [PATCH 4/4] modules/libpref: add media.ffmpeg.v4l2-request.enabled
+Date: 2026-04-27
+
+Background
+----------
+Toggle for the V4L2 stateless (request API) decode path introduced
+in patch 3/4. Defaults on for Linux, mirroring the
+`media.ffmpeg.vaapi.enabled`-style shape. Users can flip to false to
+force the existing stateful `InitV4L2Decoder` (or VAAPI / software
+fallbacks) without rebuilding.
+
+Bug 1969297.
+
+diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
+--- a/modules/libpref/init/StaticPrefList.yaml
++++ b/modules/libpref/init/StaticPrefList.yaml
+@@ -12159,6 +12159,16 @@
+   type: uint32_t
+   value: 2
+   mirror: once
++
++# firefox-fourier: route V4L2 stateless (request API) decode through
++# libavcodec's v4l2_request hwaccel (AV_HWDEVICE_TYPE_DRM). Required
++# for mainline-Linux Rockchip rkvdec / hantro / rkvdec2. On stateful
++# boards (Pi4 / vendor MPP) the codec's hw_configs lacks a DRM entry
++# and the path silently falls back to InitV4L2Decoder.
++- name: media.ffmpeg.v4l2-request.enabled
++  type: RelaxedAtomicBool
++  value: true
++  mirror: always
+ #endif # MOZ_WIDGET_GTK
+
+ # Set to true in marionette tests to disable the sanity test