Redesign TUI shared header and embed the GitHub logo as an image

[skarim]

Rework the shared gh-stack header (used by view and modify, and reused
by submit later in this stack) for a cleaner, more responsive look, and
replace the braille/ASCII Invertocat with a real image of the GitHub mark.

• Logo: embed the Invertocat PNG with go:embed and draw it via an inline-
  image protocol (kitty or iTerm2). It is image-or-nothing: when no protocol
  is available, stdout is not a TTY, or we are inside tmux/screen, no logo is
  drawn and the text falls back to the normal left padding. Detection is
  environment-based and cached so it never blocks the TUI, and a fixed kitty
  image id lets the header clear or replace the logo in place instead of
  leaving copies behind.
• Layout: place the logo in the top-left corner beside the title and version,
  with the stack-info lines left-aligned beneath it on the same left margin.
  Size the box to its content for each view so there is no trailing empty row.
• Responsiveness: hide the logo progressively — first when the viewport is too
  narrow, then a little before the rest of the header at short heights, where
  a vertical resize could otherwise leave a ghost of the inline image.
• Add unit tests for the header's responsive thresholds.

Old Design	New Design

[Copilot]

Pull request overview

This pull request redesigns the shared TUI header used across view and modify by replacing the prior ASCII/braille art with an embedded GitHub Invertocat PNG rendered via terminal inline-image protocols, while also making the header height responsive to its configured content.

Changes:

• Introduces inline-image logo rendering (kitty/iTerm2) with environment-based detection, go:embed PNG, and explicit clearing to prevent lingering kitty graphics.
• Refactors header layout/height logic so the header box sizes to its content and click/scroll computations use the computed header height.
• Adds unit tests covering viewport thresholds for hiding the logo.

Show a summary per file

File	Description
internal/tui/stackview/model.go	Uses computed header height for click/scroll math and clears the inline logo when the header is hidden.
internal/tui/modifyview/model.go	Uses computed header height for click/scroll math and clears the inline logo when the header is hidden.
internal/tui/shared/render.go	Updates click handling API to accept headerHeight instead of a showHeader boolean.
internal/tui/shared/logo.go	Adds embedded PNG logo rendering and protocol detection/clearing logic.
internal/tui/shared/header.go	Reworks header layout to support the inline logo, progressive disclosure, and dynamic content-driven height.
internal/tui/shared/header_test.go	Adds tests for logo viewport-fit thresholds and ordering vs header visibility.
go.mod	Adds dependencies needed for inline-image detection/TTY checks.
go.sum	Records checksums for newly introduced dependencies.

Copilot's findings

• Files reviewed: 7/9 changed files
• Comments generated: 3

[Lukeghenco]

Changes look good. Removing the ASCII art makes this less endearing though and removes the whimsicalness from the DX here. I wish we could just 1/2 size the character count on the ASCII art instead though 😢

[ktravers]

Very cool, never realized you could just use a png, but it makes sense now!