Fix reaction paging, composer scrolling, and icon loading

[HANCORE-linux]

Summary

• Page through all 22 reactions in DirectChat and GroupChat, five choices at a time. Preserve usage ranking and the selected reaction on the first page, freeze ordering while open, keep the final page width stable, and support arrows, Left/Right keys, and wheel input. Disabled arrows are visibly dimmed; selection and removal remain helper-bound.
• Keep long composer input inside a bounded viewport using Qt's TextArea.flickable attachment for scrolling and cursor visibility. Preserve existing send, selection, clipboard, editing, formatting, and attachment handlers.
• Register the existing packaged Material Symbols font with Qt and use explicit glyph codepoints instead of icon-name ligatures. Provide a bounded fallback and narrowly source-bound PlainText-gate exceptions. No font download, bundled font, or added shell-restart mechanism.

Related to #55. This reproduces and addresses one font-cache failure mechanism on x86_64; the reporter's ARM root cause and ARM resolution are not confirmed.

Validation

The exact committed files match the final local test and independent-review hash manifests.

• Focused reaction/composer, icon, message-action, transcript, panel, and PlainText regressions passed.
• Full local make test, local make test-ci, helper build/hardening, architecture checks, plugin validation, ShellCheck/Python, documentation checks, and whitespace checks passed.
• qmllint exited 0 with import/type warnings; it was not warning-free.
• The independent review found no remaining actionable findings and inspected all 72 rendered icons plus six full-window offscreen captures.

GitHub's required Arch headless suite must pass separately. The headless suite does not replace the local Quickshell fixtures or native acceptance.

Scope and remaining acceptance

• Helper, Service, protocol, manifest, installer/updater, and persisted message timestamps are unchanged.
• Native Omarchy 4.0.2/Wayland interaction, physical mouse/touchpad and pixel-only scrolling, IME, theme/window/monitor coverage, and ARM confirmation remain open. Offscreen events and captures are not native acceptance.
• The separate stale file-footer/path report, Omarchy 4.0.3/Shibumi host integration, and history-durability work in Sync chat history before reporting append success #56 are not included.
• No live deployment or release is included. No issue-closing keywords are used.