feat(lockscreen): add configurable grace period for passwordless unlock

[john-okeefe]

Allow unlocking without a password shortly after locking, matching hyprlock's behavior: within the configured window after the lock request, any keypress or mouse movement exceeding 5px immediately unlocks the screen.

Summary

Adds a configurable grace period to the lock screen ([lockscreen] grace_period_seconds, default 5s, range 0–60, 0 disables it entirely). While the window is open, any keypress or mouse movement beyond 5px unlocks without a password. This restores a feature that existed in the Quickshell-based Noctalia 4 lock screen (commit cbfe0aa92), reimplemented against the v5 ext-session-lock-v1 surface stack.

Motivation

The v5 lock screen always requires a password, which is friction when locking accidentally or by idle trigger and immediately returning. Hyprlock offers a short passwordless window; this PR brings v5 in line with that.

Type of Change

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

Related Issue

None — mirrors the author's v4 pr (noctalia-shell commit cbfe0aa92).

Testing

• just configure release / just build release — clean (733 targets).
• just test release — 89/90 pass. The single failure (upower_charge_limit_integration) is pre-existing: it segfaults identically on base a064c063 and is unrelated to this PR.
• noctalia config validate — clean with the new key in a real config.
• python3 tools/i18n-check.py — OK (new settings.schema.lockscreen.grace-period strings present).
• Manual runtime testing on Hyprland: lock, then keypress within the window unlocks immediately; mouse movement beyond 5px unlocks; after the window expires the password is required; grace_period_seconds = 0 always requires the password.

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

None — no visual change (single settings stepper added).

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 (Arch Linux).
• 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

• Implementation notes: the grace window uses a wall-clock timestamp comparison evaluated fresh on every key/pointer event (never cached, so it always expires on schedule); lockedAt/grace state live on LockScreen and survive WlSessionLockSurface recreation on resume; the logind resume path explicitly revokes grace as a safety net, though the wall-clock check alone would also expire it.
• example.toml documents the new key.