fix: resolve missing i18n key warnings (#9064)

## Summary

Fix multiple i18n missing key console warnings by correcting key paths
and adding missing translations.

## Changes

- **What**: 
- `ScrubableNumberInput`: Fixed references to non-existent
`g.ariaLabel.decrement`/`g.ariaLabel.increment` keys → use
`g.decrement`/`g.increment`
- `SidebarIcon`: Replaced `t()` with `st()` (safe translate) to prevent
double-translation when parent components pass pre-translated strings
- `en/main.json`: Added missing `menuLabels.Copy`, `menuLabels.Paste`,
`menuLabels.Select All` keys

## Review Focus

The `SidebarIcon` change from `t()` to `st()` is the key design
decision. `SidebarIcon` receives both i18n keys (from sidebar tabs via
`SideToolbar`) and pre-translated strings (from dedicated sidebar button
components). Using `st()` (which checks `te()` before translating)
handles both cases without warnings.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9064-fix-resolve-missing-i18n-key-warnings-30e6d73d3650816eaad3ce030f9c1d3f)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>

src/components/common/ScrubableNumberInput.vue

@@ -6,7 +6,7 @@
     <slot name="background" />
     <Button
       v-if="!hideButtons"
-      :aria-label="t('g.ariaLabel.decrement')"
+      :aria-label="t('g.decrement')"
       data-testid="decrement"
       class="h-full w-8 rounded-r-none hover:bg-base-foreground/20 disabled:opacity-30"
       variant="muted-textonly"
@@ -51,7 +51,7 @@
     <slot />
     <Button
       v-if="!hideButtons"
-      :aria-label="t('g.ariaLabel.increment')"
+      :aria-label="t('g.increment')"
       data-testid="increment"
       class="h-full w-8 rounded-l-none hover:bg-base-foreground/20 disabled:opacity-30"
       variant="muted-textonly"