Summary

Fixes the 9 pre-existing bugs catalogued in #5465 — CodeRabbit's review of #5463 surfaced them when the *_linux.go → *_gtk3.go rename made the lines visible, but the bugs themselves predate the default-flip and only affect the -tags gtk3 build path.

application_linux_gtk3.go

• run(): drop the early return nil on the file-association branch so startup handlers, common event setup, theme/power monitoring, and the GTK main loop still run after ApplicationOpenedWithFile is emitted. The surrounding else was also restructured so the "not a URL, potential file open?" debug log no longer fires after a successful file-association match. (applicationEvents is buffered cap 5, so sending the event before appRun won't block.)
• getTheme(): guard body[2] with len(body) >= 3 and use comma-ok type assertions for dbus.Variant and the inner string. Previously a malformed signal would panic.

linux_cgo_gtk3.go

• appName(): drop the defer C.free on the pointer returned by g_get_application_name() — that memory belongs to GLib internal storage; the caller must not free it. C.GoString already copies into Go memory.
• clipboardGet(): nil-check the gchar* and g_free it after copying. Previously leaked on every call.
• Calloc.String / Calloc.Free: change to pointer receivers. With value receivers append(c.pool, ...) mutated a copy, so the original pool stayed empty and Free() was a no-op — every c.String(...) allocation leaked.
• zoomIn / zoomOut: drop the -1.10 multiplier on zoomOut (which setZoom then clamped to 1.0) and use the reciprocal of zoomInFactor. Promoted the factor to a package-level const and removed the stale // FIXME comment.
• execJS: reuse the existing file-level emptyWorldName C string instead of allocating a fresh C.CString("") per call that was never freed.

menuitem_linux_gtk3.go

• setHidden: pass !hidden to widgetSetVisible — previously inverted, so hiding a menu item showed it and vice versa.
• setAccelerator: drop the leftover fmt.Println("setAccelerator", accelerator) development log.

Scope notes

• The identical Calloc value-receiver bug also exists verbatim in linux_cgo.go (the now-default GTK4 path). Intentionally left out of this PR to keep it scoped to the gtk3 tracking issue; happy to fold it in here or do a follow-up — your call.
• Item updated runtime assets #10 from the tracking issue is the processMenu processed guard, tracked separately as [v3] Menu.Update() is a no-op after first call due to processed guard in processMenu (Linux) #5464 since it affects both stacks.

Build verification

Verified with gofmt -d only — I'm on macOS and can't fully go build the -tags gtk3 cgo path locally. Worth a Linux CI pass before merge.

Refs #5465. CodeRabbit's full review: #5463 (review)

Test plan

• Build with -tags gtk3 on Linux (CI)
• Smoke-test launching a gtk3 build with a registered file association — confirm the app actually starts (window visible, startup handlers fired) instead of exiting after emitting the open-with-file event
• Smoke-test gtk3 build: zoomOut now shrinks instead of clamping to 1.0
• Smoke-test gtk3 build: menu item Hidden = true actually hides
• Confirm clipboard read in a gtk3 build still returns the expected text (and no leak under repeated reads — visible via valgrind or pmap delta if anyone wants to be thorough)

Summary by CodeRabbit

• Bug Fixes
  • Improved Linux startup so single-file arguments are correctly treated as file-open only when matching configured associations.
  • Stricter handling of system theme updates to avoid spurious changes.
  • Multiple memory-leak and allocation-tracking fixes to reduce native leaks and ensure per-window allocations are freed.
  • Safer clipboard handling when no text is present.
  • Corrected zoom scaling math.
  • Reduced unnecessary JS-evaluation allocations and removed stray debug output.