webui: settings polish (update gating, password confirm, video sleep, real time zone)

[Rust-KVM]

Requested by 4c · project thread

Summary

Before: "Update now" could be pressed without any update found; the password was entered once with no confirmation; Video sleep showed the stored duration even when sleep was disabled; the Timezone dropdown looked editable but did nothing and did not show the device's zone; the macro placeholder showed a literal 
; Mount with an empty URL did nothing.

After:

• "Update now" is disabled until Check reports an update.
• Setting or changing the password asks for it twice and rejects a mismatch.
• Video sleep shows "off" (-1) when disabled.
• Timezone shows the device's real zone, greyed out, with a note that changing it is not supported yet.
• The macro placeholder shows a real line break.
• Mount with an empty URL says "enter an image URL first".

How

• Backend: new read-only RPC getTimezone (TZ, then /etc/timezone, then the /etc/localtime symlink target, else UTC), registered in registry_builder.rs and added to app/tests/rpc_methods.txt.
• settings.rs: update_available and confirm_pw signals; video sleep reads enabled; mount URL trimmed and validated.
• settings_advanced.rs: loads getTimezone, inserts it into the list if missing, disables the select.

Testing

• cargo +nightly fmt --all (CI)
• cargo +nightly clippy --workspace --release --all-targets -- -D warnings (CI)
• cargo +nightly test --workspace (CI; covers the RPC method list)
• Frontend touched: trunk build --locked and wasm32 clippy in crates/rkvm-web (CI)
• Tested on an RK3588 device: a build merging all five round-2 branches was deployed to the NanoPC-T6; getTimezone returns UTC, 4K60 video plays, startup log has no WARN/ERROR.

Checklist

• No #[allow(...)] suppressions and no new unwrap() / expect() in production paths
• Hot paths stay zero-copy; periodic loops use MissedTickBehavior::Delay
• Shared protocol types live in rkvm-proto
• RPC method set changed → app/tests/rpc_methods.txt updated
• Module tree or build invariants changed → not applicable

Round 2 of the web UI retest fixes (minor issues). The five round-2 branches merge onto dev together without conflicts.

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https://claude.ai/code/session_01PbdzY6PfT28goR4BhqdsXR

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