[F-4] Set my reading text size

[liangricky7]

Blocked by: F-2 (#27), T4-6 (#25) · Blocks: —

(Was sprint 1 ticket 7.) A long article is readable without zooming the whole browser on every visit. This is an accessibility feature, not a cosmetic one.

• The right rail offers three sizes (small, medium, large). The selected one has a 4px accent ring
• The panel sits in the same screen position on every page
• The choice is saved through PUT /v1/me/preferences (T4-6, [T4-6] Reader text-size preference #25), so it survives across sessions and browsers, not only in this browser
• It scales the article text only. The header, labels and timestamps never change size. Someone who picks Large wants the article bigger, not the navigation

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Source: docs/frontend-tickets.md