fix(desktop): drop stale unused_mut allow

[1688mengdie]

Summary

The main window builder binding in appearance.rs carries an #[allow(unused_mut)] attribute, but the allow can never fire: the builder is unconditionally reassigned via disable_drag_drop_handler() on every platform (appearance.rs:610), with further reassignments in the macos and windows cfg blocks. With the mutability genuinely consumed on all targets, the suppression is dead weight that hides future regressions — if a later change ever removed all mutating uses of this binding, the attribute would silence the unused_mut warning that should have caught it. This PR deletes the attribute; the binding and all consumers are unchanged.

Fixes #2582

Type and Areas

Type: Bug fix

Areas: Rust core (desktop, main window lifecycle)

Motivation / Impact

An inert #[allow] on a hot path like main window creation is not cosmetic: it disables a compiler check for that binding indefinitely. Removing it restores the check's coverage at zero behavior cost — the attribute provably has nothing to suppress today (removing it surfaces no warning on any profile we can run), so the change is semantics-free while re-arming the lint for future regressions.

Verification

Scope note (assertion form): this branch is based on main, which carries one pre-existing unused_mut warning in src/apps/desktop/src/api/browser_api.rs:145 (fixed by a separate PR). The assertion for this change is therefore "zero warnings for the touched binding/file + no new warnings beyond that known pre-existing one", not a bare "0 warnings":

• cargo check --release -p bitfun-desktop --jobs 4 with this branch: exits 0; the only reported warning is the pre-existing browser_api.rs:145 one from main — appearance.rs surfaces nothing after the attribute removal.
• cargo check -p bitfun-desktop --jobs 4 (dev profile): 0 errors, 0 warnings.
• Reproduce: delete the attribute on main and rerun the release check — no new warning appears, proving the allow was suppressing nothing.
• The macos/windows cfg-block surfaces and unix builds are n/a locally (Windows machine) and are covered by the CI platform matrix.

AI-assisted change, lightly tested.

Reviewer Notes

• Deletion-only (-1 line); the binding, its reassignments, and builder.build() are untouched, so there is no behavior change on any platform.
• The value here is lint coverage: with the stale allow gone, unused_mut is armed again for this binding. If reviewers prefer, an alternative is restructuring the builder chain, but the attribute removal is the minimal, semantics-free fix.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable. (Not applicable: no user-facing change.)