feat(panels): add per-panel anchor bar for source-less panel opens

[Excelsies]

Summary

Adds an optional *_anchor_bar per panel under [shell.panel], consulted only when neither the global anchor nor a source bar resolved a bar:

shell.panel_anchor_bar  ->  source bar  ->  per-panel *_anchor_bar  ->  first enabled bar

[shell.panel]
launcher_anchor_bar = "Bottom"

Covers all six panels (launcher, clipboard, control_center, wallpaper, session, polkit) to stay symmetric with the existing *_placement / *_position families.

This is purely additive: it only fills the branch that previously fell through to "first enabled bar", so b6d1987's precedence and every existing configuration behave exactly as before.

Motivation

resolvePanelBarConfig() resolves a panel's bar from shell.panel_anchor_bar, then the opening source bar. Panels opened via a keybind, the CLI or IPC pass no sourceBarName, so with neither set the lookup falls through to "first enabled bar".

With multiple bars that means a panel whose widget lives on one bar opens attached to a different one. Concretely: a launcher widget on a bottom bar opens attached to the bottom bar when clicked, but noctalia msg panel-toggle launcher from a keybind opens it attached to the top bar.

shell.panel_anchor_bar can force a bar, but it is global and deliberately overrides the source bar for every panel (b6d1987), so it cannot fix one panel without relocating the rest.

Type of Change

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

Related Issue

Closes #4090

Testing

Commands run:

• meson setup build --buildtype=release && ninja -C build — clean build
• Recompiled the three changed translation units and checked output: no new warnings or errors
• just format (clang-format 22.1.8)
• python3 tools/i18n-check.py — OK: 892 tr keys, 7 trp base keys — all present in catalog (2555 entries).
• meson test -C build — No tests defined.

Manual testing on a two-bar setup (top bar default, bottom bar Bottom, left/right bar Side; launcher widget on Bottom), with no shell.panel_anchor_bar set and launcher_anchor_bar = "Bottom":

Action	Result
launcher via keybind / IPC	attaches to the bottom bar (surface y=774..1388, bottom bar at y=1375)
control-center via keybind / IPC	unchanged, still the top bar (y=34..674)
clicking the launcher bar widget	unchanged
all *_anchor_bar unset	behaviour identical to current main

Cross-checked the config layer: the stock 5.0.0-beta.9 build reports shell.panel.launcher_anchor_bar: unknown setting, the patched build validates it.

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

Notes on the unchecked boxes: only Hyprland 0.56.2 was available to me, so Niri and Sway are untested. Bars at top, bottom, left and right were all present and exercised, but I did not vary density settings, so I left that box unchecked rather than half-claim it. Scaling was covered across two monitors at 1.0 and 1.5.

Screenshots / Videos

The only UI surface is a new "Anchor Bar" select in each panel's settings group, next to that panel's existing Placement / Position controls, with an empty option for the default.

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

• Naming follows the existing per-panel families, so the keys read launcher_anchor_bar, control_center_anchor_bar, and so on, matching launcher_placement / launcher_position.
• panelAnchorBarForPanel() mirrors the existing openNearClickEnabledForPanel() in the same file rather than introducing a new dispatch pattern.
• Registered with stringIfNonEmptyField, so unset values are not serialised and existing configs round-trip unchanged.
• Happy to narrow this to the launcher alone, or to flip the precedence so the per-panel value outranks the source bar, if either fits the project's intent better.