launcher: add Host Mounts tab to the Machine Configuration GUI

[codewiz]

Follow-up to the HOSTFS filesys work (#159): a config-screen tab to add and
edit host-directory mounts. Independent of the routing/FilesysUnit refactor
in #172 -- this touches only src/video/* and builds on MountSpec as it
landed in #159. The two can go in either order.

What changed

• Host Mounts tab: per mount a directory (folder picker), a volume name,
  and a boot priority.
• Four editable slots (FILESYS_GUI_SLOTS). Mounts beyond that are preserved
  untouched on save rather than dropped, so a hand-written config with more
  entries survives a round trip through the GUI.
• Text entry now takes the platform layout-aware characters, so lowercase and
  underscores work in the volume-name and path boxes.
• Paths show in full, tail-clipped with a leading "..." when too long for the
  field.

Testing

• cargo test green (1378 lib + 32 + 18 + 6), cargo clippy --all-targets
  clean, cargo fmt --check clean
• Manually exercised the tab in the launcher and saved a working config

🤖 Generated with Claude Code - and then hand-edited a bit

[Copilot]

Pull request overview

This PR extends the launcher’s Machine Configuration GUI with a new “Host Mounts” tab for editing [[filesys]] host-directory mounts (directory picker, volume name, boot priority), and updates keyboard text-entry handling and path rendering to better support layout-aware characters and long paths.

Changes:

• Add a new launcher tab and fields to edit up to FILESYS_GUI_SLOTS (4) HostFS mounts, while preserving additional [[filesys]] entries beyond that on save.
• Route layout-/shift-aware text from the platform into focused launcher text fields (falling back to keycode mapping when unavailable).
• Display HostFS directory paths in full with tail-clipping (...) and use a folder picker for HostFS directory selection.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/video/launcher.rs	Adds HostFs tab/rows/fields, stores HostFS mount state, and serializes GUI-edited mounts while preserving extra [[filesys]] entries.
src/video/window.rs	Plumbs platform text into UI key handling, improves text entry for launcher edits, and adds a HostFS folder picker path.
src/video/ui.rs	Adds tail-clipping helper for long HostFS paths and uses it when rendering HostFS directory fields.
src/video/window/tests.rs	Updates UI-key-handling tests for the new ui_handle_key(..., text) signature.

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[codewiz]

Rebased on main (post-#172) and addressed the review:

• Volume-name validation (window.rs:3122): the checks were duplicated between the [[filesys]] validator and the [scsi]/[ide] drive-name parser, so I folded both into filesys::volume_name_error() and reused it in the launcher. Committing a name with ':' '/' NUL or over 30 bytes now shows the reason in the status line and keeps the field focused so it can be fixed, rather than saving a config that will not load. Net -27 lines in config.rs.
• Round-trip test (launcher.rs:966): added host_mounts_round_trip_and_keep_entries_past_the_gui_slots, covering >4 mounts, an untouched save, and a cleared slot; plus a test for the invalid-name path.
• Slot renumbering (launcher.rs:955): working as intended. HOSTFS<n> is defined as the position in the config, so clearing a slot renumbers the mounts after it exactly as deleting that [[filesys]] block from the TOML by hand would. The GUI slot labels are positional too, so the panel stays self-consistent after a reload. Making a cleared slot hold its number would need the mount to carry an explicit device name in the config, which is a separate change (and a reasonable one, if we want startup scripts to be able to rely on a stable HOSTFS number).

Tests, clippy and fmt are clean.

[LinuxJedi]

Looks great, many thanks!