Add version display and manual update checker

[TheZeekA]

Adds a version label at the bottom-right of the window. Clicking it checks the latest GitHub release; if newer, shows release notes in a modal with an explicit "Update Now" button. Nothing downloads or applies automatically - matches the discussion on issue-adjacent design questions (manual check, confirmed apply).

Update mechanism: download to temp -> verify SHA-256 against the checksum published in the release notes (aborts if mismatched) -> rename the running exe aside -> move the verified exe into place -> spawn it -> exit. Works the same for portable-exe and MSI-installed setups since both just put ZeeCrypt.exe in a normal per-user-writable folder.

Caveats, please read before merging/testing:

• This is the riskiest code from this session - it replaces its own running executable. I have no C compiler in this environment, so none of this has been built or run, only checked for Go syntax validity (gofmt) and manually traced against the giu library's actual source for API correctness.
• The right-aligned label positioning is hand-rolled (mirrors giu's own AlignRight math, since Align() has a documented bug that skips Selectable widgets) - visually unverified, may need pixel tweaks once you actually see it rendered.
• The release-notes text currently shown will just be checksums for existing releases, since build-windows.yml's release body doesn't currently include real changelog content - only checksums. Worth a follow-up to enrich that if you want nicer in-app release notes.
• Please actually test the full update flow end-to-end (ideally against a real newer release) before trusting this in production - test both a successful update and a deliberately-corrupted-checksum failure path.