feat: add light theme

[triesmon]

Description

Adds a basic light theme with a toggle in the user menu.

The selected theme is saved in localStorage instead of user settings. This allows per-device flexibility that accessibility requiring users might desire. The argument for maintaining light mode is that users with visual impairments depend on and expect an experience consistent with other websites because they've tuned their accessibility settings to help with websites that are typically in light mode. See seerr-team#1793

I also added light versions of the Seerr wordmark logos so they stay readable on light backgrounds without changing the icon colors.

How Has This Been Tested?

• Ran pnpm lint
• Ran pnpm i18n:extract
• Ran pnpm build
• Ran pnpm test
• Manually checked light/dark theme switching in Chrome and Firefox
• Manually checked theme persistence after reload
• Manually checked the sidebar logo, title text, and title card overlay in light mode

Screenshots / Logs (if applicable)

N/A

Checklist:

• I have read and followed the contribution guidelines.
• Disclosed any use of AI (see our policy)
• I have updated the documentation accordingly. N/A - no docs changes needed.
• All new and existing tests passed.
• Successful build pnpm build
• Translation keys pnpm i18n:extract
• Database migration (if required). N/A - no database changes.

AI disclosure: I used AI to help implement and test this change, then reviewed the changes myself.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Give the light logo an accessible name

When the light theme is active, theme-light.css hides .seerr-logo-dark and shows .seerr-logo-light; this second image is rendered with alt="" and aria-hidden, so the sidebar home links that consist only of the logo have no accessible name for screen-reader users. In that light-theme scenario, expose the same alt text on the visible logo instead of hiding it from assistive tech.

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