feat: add MUI theme foundation, plan panel tabs, meal type chips, sticky week layout, consistency sweep, and test updates

[carterbs]

Summary

• establish new earthy MUI theme with palette, typography, shape, shadows, and overrides
• replace PlanPanel toggle buttons with centered pill-style Tabs and keep Share menu aligned right
• swap effort emojis for meal-type chips in meal cards and streamline tests and docs
• stick weekday headers atop the meal plan, insert subtle dividers, and enforce an 8/16px spacing rhythm
• standardize meal plan labels and spacing, rely on theme tokens, and purge leftover emojis
• update tab interaction tests and document final theme decisions

Testing

• yarn test (fails: command not found: ./tools/validate/validate)

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https://chatgpt.com/codex/tasks/task_e_68bc7a875f108324887a41b1bf353986