Feat/back button nav

[Codex-Crusader]

• Navigation history stack — Added _push_nav_if_changed(), _restore_nav_state(), and _on_back_click() to dashboard.py. On every sidebar render, the current
  asset/category/custom-ticker state is compared against the previous run; if it changed, the old state (including whether news and live data were loaded) is pushed onto a
  session-state history stack capped at 20 entries. A _nav_restoring flag prevents the back action from being recorded as a new navigation event.
• Back button — top-left fixed position — Removed the back button from the sidebar. An invisible is injected via st.markdown immediately before
  the st.button call at the top of the main panel. A CSS :has() sibling selector in styles.py targets the button's wrapper div and fixes it at top: 60px; left: calc(21rem + 1rem),
  placing it just below Streamlit's header bar and to the right of the sidebar. Button inherits all retro broadsheet theme variables (gold text, dark transparent background, serif
  font).
• on_click callback pattern — The back button uses Streamlit's on_click callback rather than inline body handling. This avoids a StreamlitAPIException that occurs when writing to
  a widget-backed session state key (e.g. _custom_ticker_input) after that widget has already been instantiated in the current run. The callback fires between reruns, before any
  widgets are created.
• Selectbox keys for state restoration — Added key="_selected_category" and key="_selected_asset" to both sidebar selectboxes so the restore can write to them via session state.
  A guard clears _selected_asset if the restored value is not valid for the current category, preventing a Streamlit value-not-in-options error.
• Sidebar layout stability — Category and Asset selectboxes were previously hidden entirely when a custom ticker was active. They now always render, with disabled=True applied
  when a custom ticker is confirmed, keeping the sidebar layout stable and showing the user their last tracked selection.
• Ticker placeholder correction — The sidebar ticker lookup label and placeholder used TSMC as an example, which is not a valid Yahoo Finance symbol and caused 404 errors.
  Corrected to TSM (Taiwan Semiconductor Manufacturing Company)