[v0.3] Add ticker lookup UI to the local dashboard with input validation

[Codex-Crusader]

Parent Issue

Part of #35 — v0.3 Arbitrary Ticker Support

Depends on #36 — keyword generation must exist before the UI can call it.

Context

The current dashboard/main.py (or pulseengine/local/dashboard.py after #33) only shows the 24 hardcoded assets from config/settings.py in a dropdown. There is no way for a user to analyse a stock that isn't in that list.

This sub-issue adds a ticker lookup input to the local dashboard sidebar so users can type any ticker symbol and trigger a full signal analysis.

UI Specification

A new "Look up any ticker" section in the sidebar (below the existing asset selector) containing:

1. A text input labelled "Ticker symbol (e.g. PLTR, ARM, TSMC)"
2. An "Analyse" button (or on-enter trigger)
3. A small validation hint if the input is not a recognisable ticker

The input should:

• Accept 1–10 character strings
• Convert to uppercase automatically
• Validate against Yahoo Finance (a quick yfinance.Ticker(symbol).fast_info check is sufficient)
• Show a spinner while fetching
• On success: render the full signal analysis panel for the looked-up ticker
• On unknown ticker: show a friendly error message ("No data found for {symbol}. Check the ticker symbol and try again.")

Interaction with Existing Code

• The lookup calls generate_keywords(ticker) from [v0.3] Auto-generate keywords from company name, ticker symbol, and executive names #36 to build the keyword list
• Then calls analyse_asset(ticker, keywords=generated_keywords) from src/engine.py
• The result is rendered using the same components.py rendering functions as existing assets
• Looked-up tickers are not persisted to config/settings.py — they are session-scoped only

Acceptance Criteria

• Sidebar contains a ticker lookup input and analyse button
• Input is validated before analysis is triggered (no empty strings, no excessively long inputs)
• Unknown or invalid tickers show a user-friendly error, not a stack trace
• Valid tickers render a complete signal analysis panel (signal score, explanation, price chart, news feed)
• The existing 24-asset dropdown continues to work without regression
• No Streamlit state is leaked between ticker lookups (fresh analysis each time)
• Works correctly after the [v0.3] Create pulseengine/local/ surface with full-feature Streamlit dashboard #33 restructure (lives in pulseengine/local/)

Notes

This feature must only appear in the local dashboard (local/). In the web demo (web/), the ticker lookup section should be replaced by a "Download the local app" call-to-action (tracked in #34).