feat(message): add an option to hide the count on single-reaction chips

[Th-Underscore]

Description

Adds an opt-in setting, Hide Single Reaction Counts (Settings -> General -> Messages, default off), that hides the count numeral on reaction chips where only one user reacted - the single glyph already conveys who reacted, and single-digit aggregates mostly come from bots re-paginating. If requested, more precise functionality (only removing the number when a bot reacts to its own message) can be included instead.

When the count is hidden, the chip's padding is rebalanced (ReactionNoCount modifier) so no blank space remains where the numeral used to sit; chips with plural counts are pixel-identical to before.

Purely cosmetic, and off by default.

Tested on the desktop app, Ubuntu 24.04.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation - .changeset
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

I designed and directed the feature and tested it in the GUI; the code itself was AI-generated.

Conditionally unmounts the count <Text> when there is exactly one reaction (depending on the setting hideSingleReactionCount). Reaction.css.ts adds the padding modifier declared after the base style. The setting follows the standard boolean pattern in settings.ts, gets a SettingToggle in the Messages section of General.tsx, and is registered in settingsLink.ts focus-ID map. Tests cover both default states and the enabled state asserting the modifier class lands on the chip.