feat(theme): add per-color reset button to Theme picker (@Ammarfrfr)

[Ammarfrfr]

**Summary** Adds a small per-color reset (undo) button next to each color input in Settings → Theme → Custom that resets only that color to the currently selected preset value.

Motivation
Users often tweak individual colors and want to revert one value without losing the rest of their custom theme.

Approach
Added resetColor() in Picker which finds the current preset (getConfig.theme) and falls back to the current theme color; calls updateThemeColor() to apply.
Added an undo Button with a tooltip (balloon.text) next to the hex input and kept the existing color input/palette flow for live preview.

How to test locally

cd frontend
pnpm dev
# Open the app at the Vite URL -> Settings → Theme → Custom
# Change a color and click the undo (fa-undo) button next to that color
# Only that color should revert to the preset value

Files changed
Theme.tsx

Checklist

• Lints pass (pnpm run lint / pnpm run lint-fe)
• Type checks pass (pnpm run ts-check)
• Manual verification completed

[Copilot]

Pull request overview

Adds per-color reset control in Settings → Theme → Custom so a single tweaked color can be reverted to the currently selected preset without wiping other custom edits.

Changes:

• Add resetColor() in Picker to pull preset color (ThemesList + getConfig.theme) and apply via updateThemeColor.
• Add icon-only undo button with tooltip next to the hex input.
• Adjust picker grid columns to fit the new button.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

frontend/src/ts/components/pages/settings/custom-setting/Theme.tsx:527

• resetColor() silently no-ops if current preset not found in ThemesList (e.g. corrupted/legacy config). Elsewhere in this file ("load from preset") the same lookup shows an error; do same here so reset is not a confusing no-op.

  const resetColor = () => {
    const presetTheme = ThemesList.find(
      (theme) => theme.name === getConfig.theme,
    );
    const fallbackColor = getTheme()[props.color];
    const nextColor = presetTheme?.[props.color] ?? fallbackColor;

    updateThemeColor(props.color, nextColor);
  };