feat(lockscreen): add a password reveal toggle to the login box

[liamwh]

Summary

Adds the same eye/eye-off password reveal toggle the control-center
Wi-Fi password prompt already has (network_tab.cpp) to the lock
screen's login box — a ghost button between the password field and the
login button that flips setPasswordMode and swaps its glyph.

Motivation

The Wi-Fi prompt lets you briefly check a long password while typing;
the lock screen — where typos cost a full retype and an unlock attempt —
offers no such check. The interaction pattern already exists in-shell,
so this ports it to the one password field that lacked it.

Behaviour details:

• concealed by default; reveal is a momentary toggle, never persisted;
• the reveal state resets on every lock (setLockedState(true)), so a
  revealed password never survives into the next unlock;
• disabled while authentication is in flight, alongside the field and
  the login button;
• follows the login box's visibility (blackout, loginVisible) and
  picks up the same radius/size/glyph sizing as the login button.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Related Issue

None found; searched issues/PRs for lock-screen reveal/show-password.

Testing

• just format
• just configure (debug) + full build + meson test -C build-debug:
  92/92 OK
• Manual: code-path review only so far — I can attach a screenshot of
  the lock screen login box with the toggle before review if wanted
  (avoided locking a live session while preparing this PR).

The setPasswordMode primitive this reuses is covered by
tests/input_password_mode_test.cpp; the lock surface itself has no
unit-test scaffolding upstream (scene/wallpaper machinery), matching
the Wi-Fi prompt's reveal button, which is also untested at that layer.

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

Can provide on request (requires locking a session to capture).

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I will update end-user documentation after merge, or this PR does not change user-facing configuration or behavior.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.

Additional Notes

No new settings key: the Wi-Fi prompt's toggle is unconditional, so
this mirrors it rather than growing the login-box style surface. Happy
to add a show_reveal_password_button style flag if maintainers prefer
configurability.

Disclaimer: AI-assisted tooling was used.