feat(wails): move the Go side to v3

Phases 2 and 3 of plan 009, plus the parts of phase 1 that could not
land before them. Nothing in the tree imports wails/v2 any more; all
three lint and test configurations are green and `go build .` produces
a running binary.

The point of the migration is one file. backend/events/emit.go probed
ctx.Value("events") — a v2-*private* context key — to decide whether
emitting was safe, because runtime.EventsEmit called log.Fatalf on a
context without the runtime and took the process down with it. v3's
emit takes no context, so that is now application.Get() == nil. D1
held: events.Emit keeps its ctx as the WithSink test seam, and all 45
call sites and 7 test files are untouched.

The bootstrap splits into application.New + Window.NewWithOptions +
Run. Ten bound services implement ServiceStartup instead of being
handed a context by hand from OnStartup, which also stops ten
SetContext methods being exported as bindings. jobs.Registry and
explore.SearchIndex keep theirs — neither is bound, so converting them
would be churn for no binding removed.

Four things differed from the plan and are written up in it: GPU policy
moved to the per-window LinuxWindow options rather than surviving on
LinuxOptions; there is no OnStartup/OnDomReady option, so app-level
wiring hangs off ApplicationStarted; application.NewService is generic,
so FEBindings []any could not survive (the binding generator is a
static analyser and would have seen nothing); and the quit veto had to
be restructured, because v3's dialog answers on a callback rather than
returning the button, so ShouldQuit vetoes, asks, and quits again from
the callback.

Window state saving moves to a WindowClosing hook — the size has to be
read while the window still exists, and v3's OnShutdown has neither
context nor window. backend/logging is deleted rather than ported:
v3 takes a *slog.Logger directly, so the v2 logger.Logger adapter had
no caller left.

Phase 1's tail rides along, now that it can: the Makefile's wails
invocations, all 50 webkit2_41 sites, lefthook, both packaging recipes
and ci.yml's apt lists. v3 builds against GTK4 + WebKitGTK 6.0, which
Arch and ubuntu:24.04 both ship, so the tag is a deletion rather than
a translation.

Phase 4 is next and the branch is not usable until it lands: the app
builds, but frontend/wailsjs/ is v2's tree and nothing regenerates it,
so the frontend cannot reach the backend yet.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UDCbcCZQepnpSQYJ6SxxZm

scripts/bindings-check.sh

@@ -23,7 +23,7 @@ if [ -n "$(git status --porcelain -- "$TARGET")" ]; then
 	exit 1
 fi
 
-go tool wails generate module -tags webkit2_41 >/dev/null 2>&1
+go tool wails generate module >/dev/null 2>&1
 
 # `wails generate module` rewrites the three runtime files as 755 every
 # time.  That is not drift, so compare content only.

scripts/dev-headless.sh

@@ -12,7 +12,7 @@
 #
 #   * We run the dev *binary*, not `wails dev`.  app_dev.go parses
 #     -devserver / -assetdir / -loglevel straight from os.Args, so a
-#     `go build -tags "dev webkit2_41"` binary serves the identical
+#     `go build -tags "dev"` binary serves the identical
 #     devserver with no file watcher, no rebuild supervisor and no
 #     reload broadcast: one process, one PID, deterministic startup.
 #
@@ -131,7 +131,7 @@ echo "$YJ_HOME" >"$HOME_FILE"
 if [ "$BUILD" = 1 ]; then
 	echo "dev-headless: building frontend + dev binary..."
 	(cd frontend && pnpm install --silent && pnpm build >/dev/null)
-	go build -tags "dev webkit2_41" -o "$BIN" .
+	go build -tags "dev" -o "$BIN" .
 fi
 
 if [ ! -x "$BIN" ]; then