Improve note header controls and persist always-on-top state

[tvhdc]

Summary

• add a visible pin/unpin toggle to each note header
• keep the toolbar button and existing Always on Top context-menu item in sync
• persist the pinned state across reloads
• use distinct XApp pin and unpin symbolic icons with the same color as the other header icons
• reduce the note title size to 1em and give the header actions consistent spacing

Why

The existing always-on-top action is only available from the title-bar context menu, so it is difficult to discover. Its state is also kept only in memory and is lost when notes are reloaded or the application restarts. The format button additionally has unique padding that makes the header actions look uneven.

This change exposes the existing window-manager behavior directly in the note header and saves it with the rest of the note metadata. Notes without a pinned field remain unpinned, and unpinned notes omit the field, preserving compatibility with existing note data and manager lookups.

Validation

• python3 -m py_compile usr/lib/sticky/*.py
• git diff --check
• parsed usr/share/sticky/sticky.css with Gtk.CssProvider
• verified xapp-pin-symbolic and xapp-unpin-symbolic are available
• exercised pin/unpin state, icon switching, and backward-compatible serialization
• manually verified _NET_WM_STATE_ABOVE toggling and persistence on Linux Mint Cinnamon

The Debian package build could not be run locally because the machine is missing debhelper-compat, dh-python, and meson; the repository's build workflow should cover the package build.